The extremal inequality approach is important in its own right. Not only has it paly a key role in characterizing capacity region of the MIMO Gaussian broadcast channel with (without) secrecy [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, but also indispensable to several other vector Gaussian multi-terminal source and channel coding problems [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}.
